Polypeptides having immunological functions such as antibodies have been found to be suitable for various pharmaceutical uses. For instance, they are utilized for the alleviation of rejection reaction to renal transplantation and in pharmaceuticals as antiviral agents for RSV infection in infants and as anti-cancer agents for breast cancer. It is expected that the use of antibody-containing pharmaceutical agents will be increasingly important.
Production of an antibody using a gene encoding the antibody is carried out by culturing recombinant cells comprising a vector to which a gene encoding the antibody is introduced and then recovering the antibody produced in the culture. Such recombinant antibodies are produced in their complete form only by animal cells, and therefore, it is preferred to use animal cells for the production of recombinant antibodies.
Production of useful substances using animal cells or recombinant animal cells is widely carried out for research and industrial purposes. In a process for producing a substance by culturing animal cells, culturing is usually carried out in a medium which contains serum. However, the presence of serum is liable to cause differences among batches, which considerably affect the yield of cells and the production of substances. Therefore, use of a medium which does not contain serum is desirable in the culturing of animal cells for the production of substances.
A process of culturing the rat myeloma cell line YB2/3.0Ag30 (hereinafter referred to as Y0) in a protein-free medium is known [Cytotechnology, 17, 193 (1995)]. Also known are processes for producing polypeptides by culturing animal cells in serum-free media [Biotechnol. Prog., 10, 87 (1994); Cytotechnology, 19, 27 (1996); Japanese Published Unexamined Patent Application No. 70757/94] and a process for producing polypeptides by inoculating into a serum-free medium transformed rat cells grown in a medium containing serum and then culturing the cells in the serum-free medium (PCT National Publication No. 502377/87).
However, there has been no report on a process for producing desired polypeptides stably for a long period of time using rat cells adapted to a serum-free medium.
As a culturing method for animal cells, batch culture is mainly employed. In the batch culture, cells are inoculated into a fresh medium and cultured therein for a certain period of time. It is known that when animal cells are cultured by batch culture, the cell growth rate and the productivity of polypeptide are low because of marked deterioration in culturing conditions during the culturing, e.g., exhaustion of nutrients and accumulation of waste matters from cells. This leads to a lowering of the polypeptide concentration in the culture, thereby raising the relative concentrations of protein components other than the desired polypeptide in the culture, such as proteins derived from cells or medium. As a result, steps for separating and purifying the polypeptide become tedious and production costs are increased, which makes the process disadvantageous.